Which Reference Books Have to Be in Your Library?

An article by Donald Altschiller in the weekend edition of The Wall Street Journal named the five reference books essential for every home library:

1. World Almanac
2. Yale Book of Quotations
3. Oxford Atlas of the World
4. Merck Manual of Medical Information
5. American Heritage Dictionary of the English Language

Freakonomics Blog Asks In a heavily online-oriented world, is there still a place for printed reference volumes that provide information not available on the Web or present information more conveniently than similar online sources? Can readers of this blog suggest other print reference books that are still desirable for home libraries?